The present invention concerns toner fusing arrangements which fix xerographic toner images on copy paper by means of a heated fusing roller and an unheated counterpressure roller which can be swung towards and away from the heated fusing roller, the copy paper whose toner image is to be fused being transported through the nip between the two rollers, with the toner image itself being contacted by the peripheral surface of the heated fusing roller.
Such toner fusing arrangements are known, for example, from Federal Republic of Germany published patent application Nos. 25 28 107 and 25 29 765. These are a great deal simpler and more reliable in operation than other conventional fixing arrangements of the type wherein both of the two rollers are positively heated by means of heating elements or IR radiators located interiorly or exteriorly of the rollers. In both cases, the copying machine is not ready to perform fixing operations until the peripheral surfaces of both rollers have reached the temperatures required for a fixing operation. The peripheral surface of the unheated (or not positively heated) counterpressure roller does not as a rule come into surface contact with the peripheral surface of the positively heated fusing roller, during the warm-up or starting time interval of the fusing arrangement. Instead the peripheral surface of the unheated counterpressure roller, and in particular the part of such surface facing the heated fusing roller, heats up due to radiation of heat from the positively heated fusing roller. When now the counterpressure roller is swung towards the fusing roller for performance of a fixing operation, the differently heated sectors of the peripheral surface of the not positively heated counterpressure roller contact successive sectors of the peripheral surface of the heated fusing roller, and cool the successive sectors of the fusing roller to differing respective degrees. To prevent the toner image from being differently fused at different locations along the length of the copy paper fed through the rollers, it is necessary to heat the fusing roller to a considerably greater and more energy-consumptive extent than would otherwise be necessary; otherwise, a so-called cold-setting of toner particles may occur at longitudinally spaced locations on the transported copy sheet.